A liquid drop may spread faster on surfaces when surfactants are added. Here we show that after some time the spreading in such systems can, under certain conditions, spontaneously reverse to retraction and the droplet pulls itself back, receding from areas it has just recently wetted, elevating its center of mass in a jerklike motion. The duration from drop placement to the onset of retraction ranges from hours to less than a second primarily as a function of surfactant concentration. When the retraction is asymmetric, it results in drop motion, and when it is symmetric, the mass of the drop collects itself on its spot. This phenomenon, which was predicted theoretically in 2014, is apparently a general one for drops with surfactants; however, other factors, such as evaporation and contamination, prevented its observance so far.

Multiphase emulsions, such as drops in a continuous medium, tend to have surfactant-like impurities present at the interfaces, either naturally or introduced artificially for stability, which may influence the flow field and, hence, alter the motion of the drops through a host of different mechanisms. Here, we carry out a robust analysis to characterize multiple aspects of such interfacial phenomena by studying the settling of a drop in a quiescent viscous medium. The surface active agents are assumed to be bulk-insoluble and non-ideal, while the interface itself is assumed to have its own rheology, described by the Boussinesq–Scriven model. The diffusive fluxes of the surfactants are expressed in a thermodynamically consistent manner as proportional to the chemical potential gradient, which results in concentration dependent diffusivity. We subsequently derive semi-analytical solutions for approximately spherical drops without any other restrictions on the transport processes. Our results reveal that stresses originating from interfacial rheology tend to decrease the settling velocity and at the same time make the surfactant concentration uniform across the surface. Remarkably, this settling velocity is revealed to be independent of the choice of the free-energy isotherms and the extent of packing of the surfactants when a variable diffusivity is correctly accounted for. These insights will be helpful in better understanding of the underlying dynamics of surfactant-laden drops, having potential applications in microfluidic devices, food and pharmaceutical industries, and separation processes.

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